Carburettors comprise a passageway for the passage of combustion air provided with a portion in the form of a Venturi at the throat of which a passage for the admission of an air-gasoline emulsion opens. In certain carburettors, a centre emulsion tube is provided, which divides the combustion air passage into an inner circuit and an outer circuit, and which is placed so that its downstream end is located at the throat of the air passageway. The quantity of gasoline admitted into the air passageway depends on the depression which is produced by the inner circuit at the level of the air-gasoline emulsion intake passageway.
The centre emulsion tubes are most often constituted at the present time by a simple cylindrical tube in which the emulsion intake passageway opens laterally.
Center emulsion tubes are also known, of which the inner circuit has the form of a venturi, the emulsion intake passageway opening substantially at the throat of the Venturi. The flow of the air in the inner circuit produces at the Venturi throat of the centre emulsion tube a greater depression, which enables more gasoline to be sucked in. This type of center emulsion tube is more stable than the preceding one. By regulating the main jet upstream in order to adjust the rate of flow so that it has the correct value at normal speed, the center emulsion tube allows a more regular functioning at low r.p.m.